Ferromagnetic materials with rectangular hysteresis cycle and method for their manufacture



Jan. 31, 1961 A. PIERROT ETAL 2,970,112

FERROMAGNETIC MATERIALS WITH RECTANGULAR HYSTERESIS CYCLE AND METHOD FOR THEIR MANUFACTURE Filed April 10, 1956 A 4 Sheets-Sheet 1 a'm l i i a 2 I -H f m -H I "HG/77 C/fl *H i I L -1T IVA .2770

Jan. 31, 1961 A. PIERROT EI'AL 2,970,112

FERROMAGNETIC MATERIALS WITH RECTANGULAR HYSTERESIS CYCLE AND METHOD FOR THEIR MANUFACTURE Filed April 10, 1956 4 Sheets-Sheet 2 H fg lr 56%;

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aerated Jan. 31, 1961 A P ERROT ETAL FERROMAGNETIC MATERIALS WITH RECTANGULAR HYSTERESIS CYCLE AND METHOD FOR THEIR MANUFACTURE Filed April 10, 1956 4 Sheets-Sheet 4 2,910,112 Ice Patented Jan 1, 71961 FERROMAGNETIC MATERIALS WITH RECTAN- GULAR HYSTERESIS CYCLE AND METHOD FOR THEIR MANUFACTURE Andr Pierrot, Yves C. E. Lescroel, and Bogdan Grahowski, all of Conflans Ste.-Honorine, France, assignors to Lignes Telegraphiques and Telephoniques, Paris, France Filed Apr. 10, 1956, Ser. No. 577,327 Claims priority, application France May 3, 1955 8 Claims. (Cl. 252-625) This invention relates to ferromagnetic materials of the ferrite type, having substantially rectangular hysteresis cycles, and to their manufacture methods.

Such materials can be employed in magnetic recording devices known as memory devices, magnetic control members, magnetic amplifiers and the like. In these applications, materials according to the invention are generally used in the form of toroidal cores or at least of closed magnetic circuits without air-gap.

- Materials with a hysteresis cycle of rectangular shape are already known, particularly alloys of iron and nickel or of iron and silicon, themagnetic properties of which are most frequently rendered anisotropic either by cold rolling or by heat treatment under a magnetising field. These materials, generally speaking, have high saturation flux density and low coercive fields.

The great drawback of these metallic materials, de-

spite their usually high saturation flux density, is the low value of their resistivity, which leads to considerable eddy-current losses. These high losses result in an increase of the response time and an alteration of the hysteresis cycle, which then loses its character of rectangularity as soon as the operating frequency increases. If it be desired to employ these materials at several megacycles per second, they must be obtained in very thin sheets with a thickness of the order of a few microns, and their price immediately becomes prohibitive. Before the present invention is explained, some definitions will be given of the characteristics relating to the hysteresis cycles and other magnetic characteristics which Willbe used. i i

A substantially rectangular hysteresis cycle, plotted for a magnetising field practically reaching saturation is defined in the following terms:

B z saturation magnetic flux densityin gauss;

B,.:residua1 or remanent magnetic flux density in gauss;

H rcoercive field in oersteds;

fl= zratio of remanent fiuxdensity to saturation flux density.

Furthermore, the following terms may be used in connection with a work cycle in which the field varies between'a maximum positive value H and a maximum negative value (H i B zflux density when the field has the value H,,,, in

gauss;

B :residual or remanent flux density, in gauss;

fl =g zcoeffieient of rectangularity;

iB zfinalyalue of the flux density when the magnetising 2 field varies from a value H comprised between 1-1,, and 2H to the value R g zratio of rectangularity.

It is also possible, in some cases, to evaluate the slopes of the substantially vertical and horizontal sides of the hysteresis cycle.

The quantities:

AB -(m and AB *(rn in which AB ahd AH are small variations of the flux density and of the magnetising field in the vicinity of a given point, are respectively the slope of the curve representing the hysteresis cycle when the field passes through zero and the slope of the curve when the flux density passes through zero.

For an ideal rectangular cycle, the respective values of P and P would tend towards unity and infinity; the coefiicient of rectangularity p and the ratio of rectangularity R would tend towards unity.

The magnetic permeability n is defined as the initial permeability in the demagnetised state.

The coefiicient of eddy-current losses F expressed in ohms per henry, referred to the frequency of 800 cycles: per second is measured between 100 and 200 kilocycles per second, for a field of 2 millioersteds and at a temperature of about 20 C. for the circuits, the cross-sec tion of which is about 0.3 square centimeter.

The magnetostrictive effect may be defined by the value of the coeflicient of magnetostriction at saturation A,, which is obtained by extrapolating, for the demagnetised state, the curve of relative variation Al/l, in the applied field direction, of the length I of the sample, versus this field, plotted for very high field strengths. The response time is defined by considering two windings, having negligible time constant, placed on a t core made of the magnetic material concerned; this core a current pulse the rising time of which is very short (for example, less than 0.1 microsecond) is app ied to one of the windings and causes the magnetising field' to. pass to the value (-H the response time 1 is the time, expressed in microseconds, necessary for the voltage produced in the other winding, starting from zero, to pass through a maximum-and return to 10% of the value of this maximum.

The object of the invention is to provide magnetic materials of the ferrite type having on the one hand: substantially rectangular hysteresiscycles with a coefficient of rectangularity ,B at least equal to 0.80; and on the other hand: high electrical resistivities at least equal to 10 ohms-cm, low eddy-current loss coefiicient F,, at most equal to 0.20. i i The rectangularity of the cycle is obtained by starting from materials of the ferrite type with a negative magnetostriction coefficient, which are subjected to strains developed during a heat treatment by means of which a considerable linear shrinkage is produced, of at least 8%, and which may go up to 30%, this being one of the characteristics of the methodof the invention.

In view of their high resistivity, these materials have negligible eddy current losses, which makes it possible to use them at high frequency with very low response times 1 at most equal to microseconds.

The materials according to the invention have inductions at saturation B of the order of 2,500 to 4,500 gauss, at about 20 C., coercive fields H comprised between 1 and 4 oersteds, and coefficients of rectangularity ,B greater than 0.80.

The invention provides a method of manufacture of ferromagnetic materials of the ferrite type having a substantially rectangular hysteresis cycle, comprising preparing a homogeneous mixture of fine powders of ferric oxide, with oxides of manganese and cobalt, and if desired oxide of zinc; the molecular proportions of the various oxides in the mixture respectively being 50 to 56% for the ferric oxide; 22 to 47% for the manganese and cobalt oxides and 0 to 25% for the oxide of zinc; the ferric oxide varying between 53 and 56% when no zinc oxide is present and between 50 and 53% when 25 zinc oxide is used. The method further comprises the steps of compressing the mixture into cores and heat treating at a temperature between 1200 C. and 1300 C. in a nitrogen atmosphere containing a small percentage of oxygen, followed by slow cooling carried out in an inert atmosphere.

In the above description it must be understood that I themolecular percentage of manganese oxide is conventionally referred to the number of atoms of manganese; consequently, in the following description the manganese oxide will be conventionally represented by MnO, although, in practice, it is possible to use different oxides such as MnO Mn O and so forth.

It should be noted that the above mixture necessarily comprises a hardening element. By hardening element is meant any oxide of bivalent metal capable of forming with an oxide of trivalent metal, more particularly with iron oxide, a ferrite with a magnetostrictive coefficient of relatively high but negative value and, if necessary, modifying in an appreciable manner the constant of anisotropy K of the relatively soft ferrite in which it is in solution.

The invention will be more particularly described in the following on ferrites prepared from mixtures whose starting compositions correspond to the formula xFegO uMnO, vCoO, sZnO where x, u, v and s are the molecular percentages which satisfy the following relations and The Curie points 0;, of the final product obtained, are always higher than 150 C.

In all the following description of the present invention, the compositions indicated are starting compositions before the mixture of oxides-is reduced to. powder by grinding. The increase in the iron content, due to the wear of the grinder, being for anaverage grinder about 0.8 molecule F6203, perhundre'd' molecules. of ground material, the percentagesof Fe O indicated" after: grinding have to be increased by this quantity inorder to. obhe p r entage t; es s. after grin ina Correc- 2,970,112 it a 4 tions would have to be made if a grinder were used which wore out more slowly or more quickly.

According to the invention, in a relatively soft ferrite containing iron. manganese and, if desired, zinc, cobalt oxide may be substituted for a portion of the manganese oxide.

The relatively soft ferrites in which the substitution mentioned above is made, have a practically zero magnetostriction coefficient a In a ferrite of manganese- Zinc for which A is zero, it is necessary for a certain quantity of ferrous iron to be formed so that the ferrite Fe O may have a sufficient positive magnetostriction coefficient to cancel those of the other ferrites.

In the circumstances, the presence of a certain quantity of cobalt oxide makes possible the formation of the ferrite of cobalt which, in view of the treatment comprising slow cooling, has a definitely negative coefficient of magnetostriction.

It follows, on the one hand, that the constant of anisotropy of the ferrite is substantially increased, which results in an increase of the coercive field H The formed ferrite has, on the other hand, a definitely negative magnetostriction coefficient which is a necessary condition in order that, after suitable treatment and determined consecutive shrinkage, it may have a substantially rectangular hysteresis cycle.

It has, moreover, been noticed that the resistivity of the material is greatly increased and that the coefficients of eddy-current losses are lower than 0.20, very often even less than 0.10.

The relatively high iron content has two interesting consequences: on the one hand, an increase of the Curie point 6 and a better stability of the characteristics as a function of the temperature, and on the other hand, the saturation flux density is rather high, usually reaching values comprised between 2,500 and 4,500 gauss.

The invention will be described in more detail hereafter particularly with reference to exemplary embodiments and to the attached drawings, in which:

Figure 1 represents a practically rectangular hysteresis cycle.

a Figure 2 represents the molecular percentages of ferric oxide, as a function of the molecular percentages of zinc oxide, of a material in accordance with the invention.

Figure 3 represents the characteristics of a. material, in accordance with the invention, as a function of its content of cobalt oxide.

Figure 4 represents the same characteristics ina material of another composition.

Figure 5 represents the characteristics for another material as a function of its content of ferric oxide.

Figures 6, 7' and 8 respectively represent the hysteresis cycles corresponding to materials of different .com'positions.

- In Figure l' which represents a rectangular. hysteresis 7 cycle corresponding to a field H the flux density B =OR, the remanent flux density B, =OP, the flux density B =OS corresponding to a magnetising field i 2 and the coercive field H are indicated.

For the coefficient of rectangularity there is 22 3 B, OR

for the ratio of rectangularity It should be noted that if B :-1a, there is Bra The respective: proportions: of iron, manganese and maximum for the composition:

cobalt in the mixture of oxides used in the preparation of a material in accordance with the invention are rather critical; especially for a particular content of zinc oxide, the content of ferric oxide must not differ from the optimum value by more than 3%. On the diagram of Figure 2, in which the content s of zinc oxide is plotted as abscissa and the content x of ferric oxide as ordinate, a hatched zone is shown limited by the two curves C and C the points inside this zone indicate the contents of ferric oxide to be introduced into the starting mixture of oxides for a predetermined content of zinc oxide. I a. Figure 2 shows that when the molecular percentage of zinc oxide is of the order of 20%, the most suitable molecular percentage of ferric oxide is close to 52.5%, whileit should be about 54% when there is no zinc oxide. This optimum content of ferric oxide may vary slightly according to the molecular percentage of cobalt oxide which is present; in any case, the possible scope of variation of the molecular percentage of ferric oxide available is approximately 3%.

Figures 3 and 4 show the influence of the substitution of a certain number of C molecules for an equal number of MnO molecules upon the characteristics of e H B fora cycle corresponding to a field H, of 5 oersteds.

.Figure 3 shows, for example, that p passes through a 52.5% F6203, 28.3% MnO, 19.2% ZnO when 4 C00 molecules are substituted therein for 4 MnO molecules. Then, the maximum value of 18,, is near 0.90. In Figure 4, the same characteristics B H ,8,,, taken for a cycle corresponding to a field H of 5 oersteds, are plotted as a function of the contentof CoO molecules .which have been substituted for an equal quantity of iMnO molecules in the ferrite prepared from a mixture of the following composition:

54% Fe O 38% MnO, 8% ZnO .It will be seen that ,8,,, passes through a maximum. for a content of C00 molecules equal to 3%, the maximum value of a being near 0.86.

The curves of Figure 5 represent as a function of the content x of Fe O the variations of the characteristics B H p for starting compositions comprising con- ;stant contents of cobalt oxide and zinc oxideand respec- .jtively equal to 4% and 10%; themaximum of fi is equal to 0.87 and occurs for a value of x equal to 53.5;

the curve for 3 is rather sharp and p is higher than 0.80 for x comprised between 53.3 and 54.2; the fiux density corresponding to a field H of 10 oersteds passes through a maximum at the same time as fi METHOD OF MANUFACTURE Composition and nature of oxides employed .;For the mixtures, ferric oxide (Fe O saline oxide of manganese (Mn O oxide of cobalt of zinc (ZnO) are used.

These oxides must be pure and the mixture must not (C00) and oxide contain more than 0.5% of impurities. By impurities are L meant products such as silica (SiO sulphur, alltali jmetals (Na, K), and so forth. The ,black industrial cobalt'oxide must be preliminarilytreated, at 900 C., in order to eliminate its impurities and its humidity, and to bring it to a state near CoO.

. G nd piflThe mixture of oxides is ground'or milled in anfapf propriate device such as an iron grinder, with steel balls, usually for 12 to 48 hours, with a weight of distilled water equal to about twice the weight of the mixture.

Pressing H eat treatment The product so obtained is subjected to a heat treatment consisting of a heating lasting from 2 to 6 hours at a temperature comprised between 1,200 C. and 1,300 C. in a mixture of pure nitrogenand of 0 to 2% in volume of oxygen, followed by slow cooling carried out for about 15 hours in pure nitrogen.

In order to obtain the optimum properties, the tem-.

perature and atmosphere of the annealed products must be experimentally adjusted for each composition.

EXAMPLES The following examples show the characteristics of a' certain number'of materials obtained according to the invention.

Example 1 Figure 6 represents the hysteresis cycle taken in direct current for a maximum magnetising field H of 5 oersteds on a toroidal ferrite core having the following dimensions:

Millimeters Outer diameter 28.0 Inner diameter 15.0 Height 5.4

I. The starting composition of the material corresponded to the formula in molecular percentage:

52.5% Fe O 23.3% MnO, 5.0% C00, 19.2% ZnO The grinding was carried out for 48 hours in an iron mill with a capacity of 16 litres, containing about 3 kg.

-. of mixture, about 6 litres of water and about 20 kilograms of steel balls of different dimensions.

..The annealing was carried out at 1,240 C., for four hours, in a mixture of pure nitrogen and of 1% of oxygen in volume, and cooling took place in pure nitrogen. This material shows, for a cycle corresponding to a field H of 5 oersteds, the following characteristics:

Coercive field H oersteds 1.55 Maximum flux density B gauss 3,390 Coefiicient of rectangularity p 0.87 P,, 50

. P, 5,000 Initial permeability ,u 93 Coefiicient of eddy-current losses F 0.03

This very low coefficient of eddy-current losses ensures the stability of the hysteresis cycle as a function of the frequency, and a preservation of the characteristics indicated, as a function of the frequency, up to very high values; consequently, it renders the response time very short. This is also one of the characteristics of the invention.

Example 2 Figure 7 represents the hysteresis cycle taken in direct .current for a maximum magnetising field H of 10 oersteds, on a toroidal ferrite core, the dimensions of Millimeters Outer diameter 34.5 eInner diameter 27.3 Height 5.4

The startingcomposition of the material corresponded to the following formula in molecular percentage:

54% Fe O 41% MnO, 5% C00 This material has been treated in accordance with the same process as that of Examplel.

- "7 "It has a shrinkage of 13.8% and-shows the following properties: l 7

'Maximum flux density B gauss 2,540 Coercive field H oersteds 3 Coefficient of rectangularityffi 0.83 Coefficient of eddy-current'losses F $0.001 Initial permeability ,LL 35 i i Example Figure 8 represents the hysteresis cycle taken with difrect current for a maximum magnetising field H of 10 .oersteds on a toroidal ferrite core of the following di- "mensions:

Millimeters Outer diameter 34.4 "Inner diameter 27.4 Height 11.2

The starting composition of the material corresponded 'toth'e following formula in molecular percentage.

53.5% R3 32.5% MnO, 4% CoO,'10% ZnO this material has been treated in accordance with the same process as that in Example 1.

It hasa shrinkage of 14.0% and shows the following 'properties The following corresponds to a working fieldH of. -4 oersteds:

QMaximum flux density B 'gauss 2,420

"'Remanent fiuxdensity 'B do 12,020 Coeificient of rectangularity [3 0.84 Ratio of'rectangularity R 0.52

'There will now be obvious to those Skilled in the art many modifications and variations utilizing the principles set forth and realizing many or all of the objects and advantages of what has been described but which do not depart essentially from the spirit of the invention.

"Whatisclaimedis:

1. A process for producing "ferromagnetic materials of the ferrite type with a rectangular hysteresis cycle "such that the ratio of remanentflux densityto. maximum -flux density is at least 0.80 fora maximum .magnetising field between 5 and oersteds, comprising compressing ,ahomogeneous mixture of fine'powders ofmetallic-oxides wand subjecting the mixture .so compressed to a .heat 1 treatment carried .out for 2 to 6 hours at .a temperature between 1,200 and 1,300 C. in pure nitrogen with the addition of 0 to 2% by volume of oxygen, slowly cooling the compressed mixture for about hours .in aninert atmosphere, the said mixture being composed of ferric oxide, of manganeseand cobalt oxides and oxide of zinc wherein inthe said mixture the sum of the molecular j pe'rcentage of 'the'manganese oxide, conventionally related to the number of atoms of manganese and the *percentage of cobalt oxide is between "22 and 47%, the molecular percentage of cobalt oxide is between 1 f'and 6%, the molecular percentage of zinc is between '-0 and 25% and the molecular percentage o'f-ferric-oxide is between 50 and 56%-, said ferricoxide 'va'r'ying het'ween 53 and 56% when the .zinc .oxide is 0% and-be- -;tween 5 0 and-53% when.25% zinc oxide is used, the 'relationship of the ferric oxide to *thezinc oxide varying linearly between these values.

. 2. A process according to claim L -wherein the molecafar ip'ercentages'of the said ferric oxide, dobalt oxide and manganese oxide are respectively equal to :54, '5

.and41.

l3. Apro'cess accordlng'to claim 1, wherein :themoleeula'r "percentages of the said ferric 'oxide, cobalt oxide, zinc oxide and manganese oxide are respectively equal to*52.5, 5, .192 and 23.3. l '4. A' process according to claim 1, wherein the molecular percentages of the said ferric oxide, cobalt oxide, :zinc oxide andnnanganese oxide are respectively equal to 53.5, 4, 1.0 and 32.5. J a

5. A ferromagnetic body of a ferrite type rresulting :.from the'process of compressing a homogeneous mixture at :fine powders :of metallic oxides and subjecting "the mixture so compressed to a heat treatment carried out for 2 to 6 hours .at a temperature between .1,200"and '1,'300 C. in pure nitrogen with the addition of '0 to 2% -by volume of oxygen, slowing cooling the compressed mixture for'abo ut 15 hours in an inert atmosphere, the said mixture being composed of ferric oxide, of manganese and cobalt oxides and oxide of zinc wherein in the said mixture the sum of the molecular percentage of the manganese oxide, conventionally related to'the number-of atoms of manganese and the percentage ofcobalt oxide is between, 22 and 47, the molecular percentage of cobalt oxide is between l and 6, the molecular percentage of zinc is between Oand 25 and the molecular percentage of ferric oxide is between 50and 56%, said ferric oxide varying between 53 and 56% when the zinc oxide is 0% and between 50 and 53% when 25% zinc oxide is used, the relationship of the ferric oxide tothe 'zinc oxide varying linearly between these values-whereby "there is produced a rectangularhysteresiscycle such that the ratio of remanent flux density to maximum fiuxdensity'is at least 0180 for a maximum magnetising'field between 5 and 10 oerstedsfi i a c e 6. A ferromagnetic material according to claim 5, wherein the molecular percentages of the said ferric oxide, cobalt oxide and manganese oxide are respectively equal to 54,5 and 41. I

, .7. A ferromagnetic material according to claim 15, V

wherein the molecular percentages of the said ferric oxide, cobalt oxide, zinc oxide and manganese oxide are respectively equal to 52.5, 5, 19.2 and 23.3. I j

8. A ferromagnetic material according to claim 5, wherein the molecular percentages of the said ferric oxide, cobalt oxide, zinc oxide and manganese oxide are re.-

. spectively equal to 53.5, 4, '10a'nd 32.5.

References Cited 'in'the file of this patent UNITED STATES PATENTS 2,179,810 Brill et al. Nov. 14, 1939 2,535,025 Albrs-Schoenberg Dec. 26, 1950 2,551,711 Snoek et al. .May 8, 1951 2,565,861 Leverenz et al. Aug. 28, 1951 2,579,978 Snoek et a1. Dec. 25, .1951 2,626,445 Albers Schoenberg Jan. .27, 1953 2,636,860 Snoelcet al. Apr. 28, 1953 2,715,109 Albers Schoenberg Aug. 9, 1955 2,723,239 Harvey Nov. 8, 1955 2,882,236 Gort eret al. .Apr. 14, 1959 r 2,886,529 .Guillaud May 23,1959

- FOREIGN PATENTS 1,033,268 France Apr. 1, 1953 1,086,346 r V j1954 1,093,965 1954 1,117,385 1956 pages 49-5 8. 

5. A FERROMAGNETIC BODY OF A FERRITE TYPE RESULTING FROM THE PROCESS OF COMPRESSING A HOMOGENEOUS MIXTURE OF FINE POWERS OF METALLIC OXIDES AND SUBJECTING THE MIXTURE SO COMPRESSED TO A HEAT TREATMENT CARRIED OUT FOR 2 TO 6 HOURS AT A TEMPERATURE BETWEEN 1,200 AND 1,300*C. IN PURE NITROGEN WITH THE ADDITION OF 0 TO 2% BY VOLUME OF OXYGEN, SLOWING COOLING THE COMPRESSED MIXTURE FOR ABOUT 15 HOURS IN AN INHERT ATMOSPHERE, THE SAID MIXTURE BEING COMPOSED OF FERIC OXIDE, OF MANGANESE AND COBALT OXIDES AND OXIDE OF ZINC WHEREIN IN THE SAID MIXTURE THE SUM OF THE MOLECULAR PERCENTAGE OF THE MANGANESE OXIDE, CONVENTIONALLY RELATED TO THE NUMBER OF ATOMS OF MANGANESE AND THE PERCENTAGE OF COBALT OXIDE IS BETWEEN 22 AND 47, THE MOLECULAR PERCENTAGE OF COBALT OXIDE IS BETWEEN 1 AND 6, THE MOLECULAR PERCENTAGE OF ZINC IS BETWEEN 0 ANDF 25 AND THE MOLECULAR PERCENTAGE OF FERRIC OXIDE IS BETWEEN 50 AND 56, SAID FERRIC OXIDE VARYING BETWEEN 53 AND 56 WHEN THE ZINC OXIDE IS 0% AND BETWEEN 50 AND 53% WHEN 25% ZINC OXIDE IS UDED, THE RELATIONSHIP OF THE FERRIC OXIDE TO THE ZINC OXIDE VARYING LINEARLY BETWEEN THESE VALUES WHEREBY THERE IS PRODUCED A RETANGULAR HYSTERESIS CYCLE SUCH THAT THE RATIO OF REMANENT FLUX DENSITY TO MAXIMUM FLUX DENSITY IS AT LEAST 0.80 FOR A MAXIMUM MAGNETISING FIELD BETWEEN 5 AND 10 OERSTEDS. 